The present invention relates generally to enhanced oil recovery techniques employing in situ combustion or fire flooding, and more particularly to such recovery of petroleum from subterranean oil reservoirs characterized by relatively low permeability and temperatures wherein oxygen, ozone or a combination thereof is utilized as the combustion supporting medium for the in situ combustion.
Enhanced oil recovery procedures are utilized in various reservoir flooding and treatment programs for recovering oil from reservoirs which have previously been pressure depleted and sometimes subsequently water flooded. Fire flooding is receiving increased interest as a viable enhanced oil recovery technique. Fire flooding is practiced by injecting compressed air into an injection well and igniting the petroleum in the reservoir surrounding the injection well to establish a combustion zone or fire front. This fire front propagates into the reservoir in a radially expanding manner away from the injection well. The heat of combustion decreases the viscosity of the oil in the reservoir immediately in front of the fire front and together with the pressure of the injected air, continually forces the oil to a location in the reservoir ahead of the moving fire front. Some of the heavier petroleum components remain in the reservoir so as to provide the fuel required for the combustion process. The combustion of the petroleum in the presence of the compressed air provides a considerable quantity of water vapor which together with the gaseous carbon dioxide and nitrogen are forced into the reservoir to displace the oil reduced in viscosity away from the fire front into the reservoir. The hot water vapor and the hot gaseous products also act with the heat of combustion to reduce the viscosity of the oil in the reservoir near the fire front. The temperature of the reservoir ahead of the combustion front declines rapidly to the ambient reservoir temperature, so as to cool the warmed oil and increase the viscosity thereof which substantially decreases the mobility of the oil. This action forms a region in the reservoir containing mobile phases of oil and gas that is referred to as an oil bank. By maintaining a constant injection pressure, the fire front propagates through the reservoir and forms a plurality of mobile regions. The region nearest the fire front is formed of three phases, gases, oil and water which may be residual water in the reservoir as well as the water formed during combustion. The second region immediately removed from the first and further away from the fire front is formed of two phases with the oil and gas and provides an oil bank with the water in the first region acting as pusher or piston for moving the oil and gas through the oil formation. As the fluids are displaced from the combustion front into cooler zones in the reservoir and the mobility of the liquids decrease, there is an increase in liquid saturation which increases the resistance to gas flow from the fire front through the reservoir.
Historically, fire flooding procedures have been practiced in reservoirs which have relatively high permeabilities of greater than about 100 millidarcies (md). It was found that this relatively high permeability value is necessary for the successful practice of the in situ combustion process due to the large volumes of compressed air required to sustain the underground fire front and for venting the resulting gaseous combustion products through the reservoir. In a conventional fire flood, the fire front radially expands from the injection well to several hundred feet depending upon the spacing between the injection well and the producer well or wells. Normally, the width or height of the fire front seldom exceeds about ten feet, again depending upon the thickness of the reservoir being fire flooded. The temperature profile over the length of the fire front levels off to ambient reservoir temperatures at locations a short distance, e.g., about 24 inches, in front of the fire front. The fire front decreases the viscosity of a substantial amount of petroleum in the reservoir and under steady-state conditions forms the aforementioned oil bank which is a region of constant saturation within the rock matrix with the oil bank growing at a rate proportional to saturation.
While fire flooding techniques using compressed air have shown some success, considerable problems and difficulties are encountered such as the large volume of injected compressed air required to sustain the in situ combustion process and the large volume of nitrogen present in the combustion products which does not attribute to the recovery procedure and yet must be forced through the reservoir in order to provide for successful recovery of petroleum. Efforts to overcome these and other problems associated with using compressed air have been somewhat alleviated by employing oxygen-enriched air or pure oxygen as the combustion supporting medium. The use of oxygen requires that the materials utilized in the injection well be compatible with oxygen so as to prevent the destruction of these materials by corrosion or combustion. By employing oxygen as the combustion supporting medium only about one-fifth as much volume is required for the injection process as compared to compressed air since nitrogen which accounts for about 80% of the volume of the compressed air is eliminated. Thus, the use of the pure oxygen in the process instead of air not only drastically reduces the volume of the combustion supporting medium required but also provides a decrease in the fluid volume following combustion which generally increases the injectivity of the reservoir matrix in the combustion zone. Further, by utilizing pure oxygen as the combustion supporting medium, lower injection pressures can be used with greater well spacings. Another advantage realized by oxygen is that carbon dioxide is essentially the only gaseous product produced during the combustion process. This gaseous carbon dioxide is highly soluble in crude oil and promotes the swelling of the crude oil to enhance the reduction in the viscosity thereof for increasing the mobility of the oil through the reservoir. The gaseous carbon dioxide produced at the burn front will flow through a reservoir once the oil and water between the oil bank and the producing well have been saturated.
While oxygen has been used as the combustion supporting medium and has shown to provide many advantages over the use of compressed air in fire flooding processes, fire floods have been historically limited to use in reservoirs of relatively high permeability due to the relatively large volume of gaseous CO.sub.2 produced in the combustion process even though this volume is only about one-fifth of the volume of the gaseous combustion products produced when using compressed air. The high permeability of the reservoir is required of reservoirs having ambient temperatures greater than about 88.degree. F. since even though the volume of gaseous CO.sub.2 is considerably less than that of the gaseous combustion products using compressed air, the permeability of the reservoir must be sufficient to allow for the gaseous CO.sub.2 to be displaced through the reservoir formation ahead of the fire front. Fire floods have not been shown to be a practical recovery process in reservoirs of low permeability, i.e., less than about 100 md, where the reservoir temperatures arc less than about 88.degree. F., especially the relatively cold (60.degree.-75.degree. F.) and low permeability (about 20 md) reservoirs such as in the Appalachian region.